This invention relates to the field of lasers and more specifically to the field of injection lasers.
Present silica-based optical fibers can be fabricated to have a loss in the 1.3-1.6 micron wavelength region which is an order of magnitude lower than the loss occurring at the 0.85 micron wavelength of present lightwave communications systems, e.g., 1/4 db/km versus 2-3 db/km. Furthermore, these fibers can be fabricated to have a transmission delay distortion in the 1.3-1.6 micron wavelength region which is two orders of magnitude lower than the transmission delay distortion at 0.85 microns, e.g., 1-2 ps/km nm versus 100+ ps/km nm. Thus, the dispersion-limited transmission distance for high bit rate lightwave communications systems can be maximized by using a single-frequency, i.e., single-longitudinal-mode, injection laser generating output at the 1.55 micron wavelength where the fibers have minimum loss. For these reasons, present efforts in the development of lightwave communications systems are aimed at the wavelength region between 1.3 and 1.6 microns instead of at the wavelength region surrounding 0.85 microns.
InGaAsP injection lasers produce output in the desired 1.3-1.6 micron wavelength region. However, typical single-resonator InGaAsP injection lasers have a laser cavity length in the 250-300 micron range. This results in mode spacing between 6 and 9 Angstroms. Since the gain spectral width of InGaAsP injection lasers is approximately 250-300 Angstroms, there are more than 30 longitudinal modes under the gain spectral width of a 250 micron long laser. Thus, the gain difference between modes is small and mode discrimination between the main mode and side modes is poor in InGaAsP injection lasers.
The injection laser, like all other oscillators, is perturbed by internal random processes which cause its output to fluctuate. One example of this, known as mode-partition-noise, is the fluctuation at turn-on in the relative intensities of various laser modes while the total output power of the laser remains fixed. Mode-partition-noise is a consequence of random fluctuations in the photon densities of the various modes at the moment lasing threshold is reached. If the main mode photon density is not the largest at that instant, another mode will build up first. In a communication system employing such a laser, mode-partition-noise can combine with dispersion in the transmission medium to produce random distortion of the received signal, thereby degrading system performance. Since the distribution of mode partition fluctuations is exponential rather than Gaussian, fluctuations large enough to cause bit error to occur result in intolerably high rates. For example, the main mode intensity drop-out due to the mode partition fluctuations can be related to the error rate in the following way. If the laser is modulated with a bit rate equal to the main mode intensity drop-out duration (approximately 1 nanosecond), about half of the drop-out events will cause error in a system with high dispersion. Thus, a drop-out rate of 1 per second will cause an error rate of 10.sup.-9 at 1 Gbit/s. Furthermore, because mode partition fluctuations are a low-frequency phenomena, lasting a few nanoseconds, they cannot be reduced by averaging in high-bit-rate (1000-MB/s) systems.
Mode-partition-noise is inherent in any laser with more than one resonant mode, e.g., the typical InGaAsP injection laser described above, and can be expected to some degree in all conventional Fabry-Perot lasers. However, the mode-partition-noise impairment does not exist when an ideal single-longitudinal-mode (SLM) laser is used, because all the power produced by the SLM laser exists in only one mode. However, in practice, there are always unwanted vestigial side modes in any laser and SLM lasers can only be approximate.
Several structures, aimed at providing single-frequency operation, have been proposed and demonstrated. These include distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers, lasers with an external cavity, injection-locking lasers, short-cavity lasers and coupled-cavity lasers. Unfortunately, these laser structures are either difficult to fabricate, difficult to operate or require external elements which are sensitive to mechanical vibration.